Radio-frequency mixer arrangements are used, by way of example, in vector modulators in order to convert a baseband signal which has undergone digital or analog modulation to a transmission frequency level. A vector modulator is constructed using integrated circuitry, normally with Gilbert multiplier cells.
An integrated multiplier circuit which is suitable for use in vector modulators in radio transmission appliances is described by way of example in the document DE 101 32 802 A1. The multiplier circuit in this document has two input terminal pairs which are used for supplying a first and a second signal which are to be multiplied. In this case, the resultant frequency of the multiplied signals is obtained from the sum of, or from the difference between, the input signal frequencies. One of the two inputs of the known multiplier circuit has voltage/current converters which are connected to the multiplier input via current mirrors. This improves the noise characteristics.
Besides good noise characteristics, however, improved linearity is demanded of vector modulators, particularly in respect of such modulation methods as take account of the constantly rising data rates.
The linearity of a Gilbert multiplier cell is proportional to the product of operating current and feedback resistances in the differential amplifiers. To achieve sufficient linearity and high gain, the differential amplifiers normally need to be supplied with a correspondingly large operating current. The feedback resistances and the operating current are the primary cause of the noise at the modulator output, besides the oscillator's phase noise. In addition, the predominant use of such integrated transmission arrangements in mobile radios, which are battery-powered, means that a low drawn current is desirable.